This invention relates to an active filter circuit that uses capacitors and operational amplifiers.
FIG. 8 is a schematic showing the construction of a general active filter circuit.
The low pass filter LPF shown in FIG. 8, which is one example of an active filter, is constructed of an operational amplifier AMP, capacitors C1, C2, and resistors R1, R2.
The resistor R1 and the resistor R2 are connected in series between the input terminal Tin to which the input signal Vin is input and the non-inverted input (+) for the operational amplifier. The capacitor C1 is connected between the non-inverted input (+) of the operational amplifier AMP and the reference potential GND.
The capacitor C2 is connected between the connection point for the resistors R1, R2 and the inverted input (xe2x88x92) of the operational amplifier input. The inverted input (xe2x88x92) of the operational amplifier AMP is connected to the amplifier output, and negative feedback is applied. The output of the operational amplifier AMP is connected to the output terminal Tout, and the output signal Vout is taken out from the output terminal Tout.
With this conventional filter circuit, there is no capacitance voltage dependency and capacitors with a large capacitive value are necessary. In forming these capacitors on the same wafer as the operational amplifier in the semiconductor manufacturing process, a dedicated process was necessary for the purpose of capacitor formation. Because of this, the number of processes for the semiconductor manufacturing process was large, and the process costs became high.
The purpose of this invention is to offer a filter circuit in which the capacitor dedicated surface area is small, and a dedicated process just for capacitor formation is unnecessary.
In the filter circuit of this invention, for example, in the case of a general-purpose logic mixed loading and the like, an insulated gate field effect transistor is used as a filter circuit that is housed in the semiconductor device, the source and drain of the insulated gate field effect transistor MOS transistor are connected in common, and this is used as the capacitor by using the gate insulating film. At that time, this is a configuration in which the channel for the transistor is not ordinarily formed. The capacitor value is not fixed but shows a voltage dependency in which the capacitor value during low voltage is extremely small. In order to solve this problem, in this invention, a DC bias means is provided.
In other words, the filter circuit related to this invention has a first operational amplifier, a plurality of resistors that are electrically connected to the above-mentioned first operational amplifier, a plurality of capacitors that are electrically connected to the above-mentioned first operational amplifier, wherein in each the source and drain of an MOS transistor are connected and become one of the electrodes, and the gate of the said MOS transistor becomes the other electrode, a DC bias means that applies a prescribed DC bias between the electrodes of the above-mentioned capacitors, and the above-mentioned first operational amplifier, the above-mentioned resistors, the above-mentioned capacitors, and the above-mentioned DC bias means are formed on the same semiconductor substrate.
This filter circuit can be a low pass filter or a high pass filter.
A DC bias means can be provided at the first capacitor and the second capacitor, respectively, but preferably, one of those can be eliminated by means of increasing the input signal level a prescribed amount.
In a filter circuit with this type of construction, the capacitive value per unit surface area can be large due to the fact that the capacitor is formed by using a thin high-quality gate insulating film, and moreover, it can be formed simultaneously with another insulating gate field effect transistor.
Also, due to the fact that a prescribed DC bias voltage can always be applied to each capacitor by means of a DC bias means, the capacitive value of a capacitor using a gate insulating film does not show a voltage dependency.